1. Field of the Invention
The present invention generally relates to a digital-to-analog converter circuit utilizing a resistance-string-type D/A converter.
2. Prior Art
FIG. 3 shows a circuitry including a conventional resistance-string-type D/A converter (or ladder-network DAC). A main part 21 of the D/A converter (hereinafter, simply referred to as a D/A conversion part 21) comprises a string of resistors, which are connected in series, and CMOS switches (where `CMOS` is an abbreviation for Complementary Metal-Oxide Semiconductor). Herein, the switches are provided in parallel such that each switch is connected with a connection node at which two resistors are connected together. Power-supply voltage is applied between both edges of the resistance string. Each switch is turned on or off in response to input digital data. Hence, digital-to-analog conversion is carried out. An output of the D/A conversion part 21 is supplied to an amplifier circuit 23, containing a filter (not shown), through a voltage follower 22 made with an operational amplifier.
There is provided a center-tap terminal 24 which is located at the center of the resistance string. Hence, the output of the D/A conversion part 21 is extracted from the center-tap terminal 24 through the switch. Therefore, the output of the D/A conversion part 21 varies in voltage on the basis of the potential at the center-tap terminal 24; hereinafter, this potential will be called a reference voltage V.sub.REF. The reference voltage V.sub.REF is used as the reference, based on which the amplifier circuit 23 operates.
In the conventional circuitry for the resistance-string-type D/A converter, the reference voltage V.sub.REF is obtained as one fraction of the total voltage across the resistance string according to the principle of the resistance-type potential divider. For this reason, the reference voltage V.sub.REF may vary in response to the variation of the power-supply voltage. Thus, the output voltage should also vary. In addition, the reference voltage V.sub.REF is produced inside of the D/A converter circuit. This raises a problem that when the amplifier circuit 23 is designed to operate with another reference voltage, it is difficult to perform a signal transmission between the D/A conversion part and amplifier circuit. For the same reason, when combining multiple D/A converters, multiple reference voltages should be produced. This raises a problem that the desired system cannot be configured using the multiple D/A converters conventionally known.